III-Nitride power semiconductor devices are commercially desirable due to their high band gap and high current carrying capabilities.
FIG. 1 illustrates an example of a typical high electron mobility transistor (HEMT), which is formed using a heterojunction III-nitride-based semiconductor die. Specifically, a HEMT according to the prior art includes a first III-nitride semiconductor body 10, which may be composed of, for example, undoped GaN, and a second III-Nitride semiconductor body 12, which may be composed of, for example, N-type AlGaN, disposed over first semiconductor body 10. First semiconductor body 10 may be formed over a transition body 8 composed, for example, of AlN, which is itself formed over substrate 6. Substrate 6 is preferably formed from Si, but may be formed from SiC, Sapphire, or the like. Alternatively, substrate 6 may be formed from a bulk III-Nitride semiconductor (e.g. bulk GaN) which is compatible with first semiconductor body 10, in which case transition body 8 may be eliminated.
As is well known, the heterojunction of first semiconductor body 10 and second semiconductor body 12 results in the formation of a conductive region usually referred to as a two dimensional electron gas or 2DEG 14. Current may be conducted between a first ohmic contact 16 (which is ohmically coupled to second semiconductor body 12), and second ohmic contact 18 (which is also ohmically coupled to second semiconductor body 12) through 2DEG 14.
A conventional HEMT, such as the one seen in FIG. 1, is a normally ON device. In many applications it is desirable to turn the device off or have a normally OFF device. Thus, a gate electrode 20 may be disposed between first ohmic contact 16 and second ohmic contact 18. Gate structure 20 includes at least a gate electrode which may be electrically insulated by gate insulation 21 and thus capacitively couple to second semiconductor body 12. The application of an appropriate voltage to gate electrode 20 causes the interruption of 2DEG 14 thereby turning the device OFF. Thus, to operate a conventional HEMT as a normally OFF device requires the continued application of a voltage to gate electrode 20, which is not desirable as it consumes more energy, and may also require a more complicated drive circuitry compared to a normally OFF device.